910 A TREATISE ON METAMORPHISM. 



sediments have somewhat the same homogeneity of structure that igneous 

 rocks possess. Where there is any considerable sorting of the sedimentary 

 materials, so that they are arranged in layers of different composition, this 

 fact tends to prevent their change to forms which appear similar or identical 

 with metamorphosed igneous rocks. 



(3) A third case which has occasioned difficulty is that of an igneous 

 rock which has been intruded by another igneous rock in a very compli- 

 cated fashion, so that the intrusive contains very many fragments of the 

 intruded rock. These fragments are oftentimes partly absorbed, and thus 

 take on a pseudo-conglomeratic aspect. While it is comparatively easy in 

 most cases to separate such a rock from a true conglomerate when in the 

 unalterated condition, the difficulty of the discrimination increases in 

 proportion as the metamorphism is advanced. The difficulty may be 

 appreciated from the fact that cases are known where immetamorphosed 

 pseudo-conglomerates of intrusion and true conglomerates which happen to 

 be side by side have been confused by geologists. In various cases included 

 blocks in gneissoid granites have been taken as evidence of the sedimentary 

 character of these rocks. This mistake has been made in reference to the 

 Cottonwood granite of the Wasatch, which contains rounded fragments, or 

 else segregations in situ, which have distinctly the aspects of bowlders. 

 Similar phenomena have led to like conclusions in the Lake Superior 

 region. In some instances the intruded rock and included blocks are 

 schist. In such cases parallel injections are also likely to occur. Thus we 

 have a pseudo-conglomerate, below which are alternating layers of different 

 kinds. These phenomena in the Lake Superior region have led certain 

 geologists to the conclusion that the explanation of the phenomena is 

 downward progressive metamorphism ; but it has been shown that the 

 phenomena are due to intrusion. These difficulties are greatly increased 

 where the rocks have been mashed subsequent to the intrusions. 



(4) Another case in which it may be difficult to determine the origin 

 of the metamorphosed rocks is that of the porphyrinic schists. By the 

 mashing of a porphyritic igneous rock the matrix may become schistose. 

 The phenocrysts may become rounded by the interior movement, and 

 closely resemble residual fragmental material. In many cases the porphy- 

 ritic schists of sedimentary origin are derived from material which contains 

 large grains or crystals in a fine-grained matrix. The matrix may be 



